Add : D-108, Sec-2, Noida (U.P.), Pin - 201 301
Add : D-108, Sec-2, Noida (U.P.), Pin - 201 301
Email id : [email protected]
Email id : [email protected]
Call : 09582948810, 09953007628, 0120-2440265
Call : 09582948810, 09953007628, 0120-2440265
CLIMA
CLIMA
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Atmosphere is a thick gaseous envelope that Atmosphere is a thick gaseous envelope that surrounds the earth and extends thousands of surrounds the earth and extends thousands of kilometers above the earth's surface. Much of the kilometers above the earth's surface. Much of the life on the earth exists because of the atmosphere life on the earth exists because of the atmosphere otherwise the earth would have been barren. In otherwise the earth would have been barren. In fact, atmosphere directly or indirectly influences fact, atmosphere directly or indirectly influences the vegetation pattern, soil type and topography the vegetation pattern, soil type and topography of the earth.
of the earth.
Compos
Composii tt ii on of ton of t he he AtAt mosphmospheere re
The atmosphere is a mixture of several gases. The atmosphere is a mixture of several gases. It contains huge amount of solid and liquid It contains huge amount of solid and liquid particles collectively known as aerosols. Pure dry particles collectively known as aerosols. Pure dry air consists mainly of Nitrogen, Oxygen, Argon, air consists mainly of Nitrogen, Oxygen, Argon, Carbon Dioxide, Hydrogen, Helium and Ozone. Carbon Dioxide, Hydrogen, Helium and Ozone. Besides, water vapour, dust particles, smoke, salts, Besides, water vapour, dust particles, smoke, salts, etc. are also present in the air in varying quantit etc. are also present in the air in varying quantities.ies.
The chemical composition of atmosphere up The chemical composition of atmosphere up to an altitude of about 90 km is uniform in terms to an altitude of about 90 km is uniform in terms of major gases-Nitrogen and Oxygen. This layer of major gases-Nitrogen and Oxygen. This layer is called
is called Homosphere.Homosphere. Above 90 km, theAbove 90 km, the proportion of the gases changes with progressive proportion of the gases changes with progressive increase in the proportion of lighter gases. This increase in the proportion of lighter gases. This layer is known as
layer is known as Heterosphere.Heterosphere.
Nitrogen and Oxygen comprise 99% of the Nitrogen and Oxygen comprise 99% of the total volume of the atmosphere. But Nitrogen does total volume of the atmosphere. But Nitrogen does not easily enter into chemical union with other not easily enter into chemical union with other substances. It serves mainly as an agent of dil substances. It serves mainly as an agent of dilutionution and remains chemically inactive. Oxygen and remains chemically inactive. Oxygen combines with all the elements easily and is most combines with all the elements easily and is most combustible. Carbon dioxide constitutes a small combustible. Carbon dioxide constitutes a small percentage of the atmosphere. It can cause the percentage of the atmosphere. It can cause the lower atmosphere to be warmed up by absorbing lower atmosphere to be warmed up by absorbing heat from the incoming short wave solar radiation heat from the incoming short wave solar radiation from the sun and reflecting the long wave from the sun and reflecting the long wave terrestrial radiation back to the earth's surface. terrestrial radiation back to the earth's surface. Carbon dioxide is utilized by the green plants in Carbon dioxide is utilized by the green plants in the process of photosynthesis.
the process of photosynthesis.
Water vapour and dust particles are the Water vapour and dust particles are the important variables of weather and climate. They important variables of weather and climate. They are the source of all forms of condensation and are the source of all forms of condensation and principal absorbers of heat received from the sun principal absorbers of heat received from the sun and radiated from the earth. Water vapour and radiated from the earth. Water vapour comprises 3-4% of the total volume of air. comprises 3-4% of the total volume of air.
However, the amount of water vapour present However, the amount of water vapour present in the atmosphere decreases from the equator in the atmosphere decreases from the equator towards the poles. Nearly 90% of the total water towards the poles. Nearly 90% of the total water vapour lies below 6 km of the atmosphere. vapour lies below 6 km of the atmosphere.
S
Stt ructure of tructure of t he he AtAt mosphmospheere re
The atmosphere consists of almost concentric The atmosphere consists of almost concentric layers of air with varying density and temperature. layers of air with varying density and temperature.
a) Troposphere: a) Troposphere:
LoLowewest lst layayer oer of thf the ate atmomospspheherere.. •
• The The heiheighght of t of trotroposposphphere ere is 1is 16 km 6 km thithickck over the equator and 10 km thick at the over the equator and 10 km thick at the poles.
poles. •
• All All weaweathether pr phenhenomomena ena are are conconfinfined ed toto troposphere (e.g. fog, cloud, frost, rainfall, troposphere (e.g. fog, cloud, frost, rainfall, storms, etc.)
storms, etc.) •
• TemTemperperatuature dere decrecreasases wes with ith heiheighght in tht in thisis layer roughly at the rate of 6.5° per 1000 layer roughly at the rate of 6.5° per 1000 metres, which is called
metres, which is callednormal normal ll apsapse e ratrat ee..
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• UppUpper ler limiimit of t of the the trotroposposphephere ire is cs callalleded
tropopause
tropopause which is about 1.5 km. which is about 1.5 km.
b) Stratosphere: b) Stratosphere:
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• The The strstratoatospspherhere is me is morore or le or less ess devdevoioid of d of major weather phenomenon but there is major weather phenomenon but there is circulation of feeble winds and cirrus circulation of feeble winds and cirrus cloud in the lower stratosphere.
cloud in the lower stratosphere.
CLIMATOLOGY
CLIMATOLOGY
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N Niittrrooggeenn 7788..008 8 %% O Oxxyyggeenn 2200..994 4 %% A Arrggoonn 00..9933%% C
Caarrbboon n ddiiooxxiiddee 00..0033%% N Neeoonn 00..00001188%% H Heelliiuumm 00..00000055%% O Ozzoonnee 00..0000000066%% H Hyyddrrooggeenn 00..0000000055%%
EARTH'S ATMOSPHERE
EARTH'S ATMOSPHERE
CHRONICLE
CHRONICLE
IAS ACADEMY
IAS ACADEMY
A CIVIL SERVICES CHRONICLE INITIATIVE A CIVIL SERVICES CHRONICLE INITIATIVE
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• Jet aircrafts fly through the lower stratosphere because it provides conducive flying conditions.
• Ozone layer lies within the stratosphere mostly at the altitude of 15 to 35 km above earth's surface.
• Ozone layer acts as a protective cover as it absorbs ultra-voilet rays of solar radiation.
• Depletion of ozone may result in rise of temperature of ground surface and lower atmosphere.
• Temperature rises from -60°C at the base of the stratosphere to its upper boundary as it absorbs ultra-voilet rays.
• Upper limit of the Stratosphere is called
stratopause. c) Mesosphere
• Mesosphere extends to the height of 50-90 km.
• Temperature decreases with height. It reaches a minimum of -80°C at an altitude of 80-90 km
• The upper limit is called mesopause .
d) Thermosphere
• It lies at 80 km to 640 km above the earth's surface.
• It is also known as ionosphere.
• Temperature increases rapidly with increasing height.
• It is an electrically charged layer. This layer is produced due to interaction of solar radiation and the chemicals present, thus disappears with the sunset.
• There are a number of layers in thermosphere e.g. D-layer, E-layer, F-layer and G-F-layer.
• Radio waves transmitted from earth are reflected back to the earth by these layers.
e) Exosphere
• This is the uppermost layer of the atmosphere extending beyond the ionosphere.
• The density is very low and temperature becomes 5568°C.
• This layer merges with the outer space.
A bout I onosphere
At heights of 80 km (50 miles), the gas is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The number of these free electrons is sufficient to affect radio propagation. This portion of the atmosphere is ionized and contains plasma which is referred to as the ionosphere. In plasma, the negative free electrons and the positive ions are attracted to each other by the electromagnetic force, but they are too energetic to stay fixed together in an electrically neutral molecule.
The Ultraviolet (UV), X-Ray and shorter wavelengths of solar radiation ionizes the atmosphere. In this process the light electron obtains a high velocity so that the temperature of the created electronic gas is much higher (of the order of thousand K) than the one of ions and neutrals. The reverse process to Ionization is recombination, in which a free electron is "captured" by a positive ion, occurs spontaneously. The balance between these two processes determines the quantity of ionization present.
Ionization depends primarily on the Sun and its activity. The amount of ionization in the ionosphere varies greatly with the amount of radiation received from the Sun. Thus there is a diurnal (time of day) effect and a seasonal effect. The ionosphere is broken down into the D, E and F regions.
The D region is the lowest in altitude, it doesn't have a definite starting and stopping point, but includes the ionization that occurs below about 90km. Ionization here is due to Lyman series-alpha hydrogen radiation at a wavelength of 121.5 nanometres (nm) ionizing nitric oxide (NO). In addition, with high Solar activity hard X-rays (wavelength < 1 nm) may ionize (N2, O2). Recombination is high in the D layer, the net ionization effect is low, but loss of wave energy is great due to frequent collisions of the electrons. As a result high-frequency (HF) radio waves are not reflected by the D layer but suffer loss of energy therein.
The E region peaks at about 105 km. It absorbs soft x-rays. Normally, this layer can only reflect radio waves having frequencies lower than about 10 MHz. At night the E layer rapidly disappears because the primary source of ionization is no
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The F layer or region, also known as the Appleton layer extends from about 200 km to more than 500 km above the surface of Earth. It is the densest point of the ionosphere, which implies signals penetrating this layer will escape into space. At higher altitudes the amount of oxygen ions decreases and lighter ions such as Hydrogen and Helium become dominant, this layer is the topside ionosphere. Here extreme ultraviolet (UV, 10-100 nm) solar radiation ionizes atomic oxygen. The F layer consists of one layer at night, but during the day, a deformation often forms in the profile that is labeled F1. The F2 layer remains by day and night responsible for most skywave propagation of radio waves, facilitating high frequency (HF, or shortwave) radio communications over long distances.
Actually when a radio wave reaches the ionosphere the electric field in the wave forces
the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio-frequency energy is given up to this resonant oscillation. The oscillating electrons will then either be lost to recombination or will re-radiate the original wave energy. Total refraction can occur when the collision frequency of the ionosphere is less than the radio frequency, and if the electron density in the ionosphere is great enough.
The critical frequency is the limiting frequency at or below which a radio wave is reflected by an ionospheric layer at vertical incidence. If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal. On a more practical note, the D and E regions reflect AM radio waves back to Earth.
CONCEPT OF WEATHER AND CLIMATE
Eart h's cl i mat e has been changi ng for bi l l i ons of years
Climatologists have used various techniques and evidence to reconstruct a history of the Earth's past climate. They have found that during most of the Earth's history global temperatures were probably 8 to 15 degrees Celsius warmer than today. In the last billion years of climatic history, warmer conditions were broken by glacial periods starting at 925, 800, 680, 450, 330, and 2 million years before present.
The period from 2,000,000 - 14,000 B.P. (before present) is known as the Pleistocene or Ice Age. During this period, large glacial ice sheets covered much of North America, Europe, and Asia for extended periods of time. The extent of the glacier ice during the Pleistocene was not static. The Pleistocene had periods when the glacier retreated (interglacial) because of warmer temperatures and advanced because of colder temperatures (glacial). During the coldest periods of the Ice Age, average global temperatures were probably 4 - 5 degrees Celsius colder than they are today.
By 5000 to 3000 BC average global temperatures reached their maximum level during the Holocene and were 1 to 2 degrees Celsius warmer than they are today. Climatologists call this period the Climatic Optimum. During the Climatic Optimum, many of the Earth's great ancient civilizations began Weather refers to the sum total of the
atmospheric conditions in terms of temperature, pressure, wind, moisture etc of a given place and time. It is very dynamic as it may change several times even in a day. Weather changes each day because the air in our atmosphere is always moving, distributing energy from the Sun. In most places in the world, the types of weather events also vary throughout the year as season's change. Climate on the other hand, is the average weather condition or atmospheric condition of a region over a considerable period of time. The
regional cli mat eis the average weather in a place over more than thirty years. The climate of a region depends on many factors including the amount of sunlight it receives, its height above sea level, the shape of the land, and how close it is to oceans. Since the equator receives more sunlight than the poles, climate varies depending on distance from the equator.
However, Global cli mate is a description of the climate of a planet as a whole, with all the regional differences averaged. Overall, global climate depends on the amount of energy received by the Sun and the amount of energy that is trapped in the system. These amounts are different for different planets. Scientists who study Earth's climate and climate change study the factors that affect the climate of the whole planet. While the weather can change in just a few hours, climate changes over longer time-frames.
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and flourished. In Africa, the Nile River had three times its present volume, indicating a much larger tropical region.
From 3000 to 2000 BC a cooling trend occurred. This cooling caused large drops in sea level and the emergence of many islands (Bahamas) and coastal areas that are still above sea level today. A short warming trend took place from 2000 to 1500 BC, followed once again by colder conditions. Colder temperatures from 1500 - 750 BC caused renewed ice growth in continental glaciers and alpine glaciers, and a sea level drop of between 2 to 3 meters below present day levels. During the period from 750 BC - 800 AD warming up of atmosphere took place. During the time of Roman Empire (150 BC - 300 AD) a cooling began that lasted until about 900 AD. At its height, the cooling caused the Nile River (829
AD) and the Black Sea (800-801 AD) to freeze. From 1550 to 1850 AD global temperatures were at their coldest since the beginning of the Holocene. Scientists call this period the Little Ice Age. During the Little Ice Age, the averages annual temperature of the Northern Hemisphere was about 1.0 degree Celsius lower than today. During the period 1580 to 1600, the western United States experienced one of its longest and most severe droughts in the last 500 years. Cold weather in Iceland from 1753 and 1759 caused 25% of the population to die from crop failure and famine.
The period 1850 to present is one of general warming. Many scientists believe the warmer temperatures of the 20th and 21st centuries are being caused by the human enhancement of the
Earth's greenhouse effect.
The source of all energy on the earth is the Sun. The Sun acts as a great engine that drives winds, ocean currents, exo-genetic or denudation processes and sustains life on the earth. The energy radiated from the Sun comes from the nuclear fusion process taking place in its core, where the temperature is about 15,000,000ºC. The solar energy is transmitted in the form of short wave electromagnetic radiations. They travel at the speed of light (about 2, 98,000 km per second).
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum.
On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.
Factors affecting the distribution of insolation:
• Distance between the Earth and the
Sun-The solar output received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the
earth and the Sun. During its revolution around the Sun, the earth is farthest from the sun (152 million km) on 4th July. This position of the earth is called aphelion . On
3rd January, the earth is nearest to the sun (147 million km). This position is called
perihelion. Therefore, the energy received
by the earth on 3rd January is slightly more than the amount received on 4th July. • Angle of inclination of the sun's rays- It
depends on the latitude of a place. The higher the latitude the less is the angle it makes with the surface of the Earth, resulting in slanting Sun rays. Hence, the area covered by slanting rays is always higher than that of the vertical rays. Thus, the energy gets distributed in a larger area and the net energy received per unit area decreases. Thus the amount of insolation received at the Earth's surface decreases from equator towards the poles. The total amount of insolation received at the equator is about four times higher than that of the poles.
• Length of the day- During the summer season days are longer than the night. The situation is reversed in the winter season. The longer the day, the more is the insolation received by earth.
• Altitude of land- Insolation heats up the land which in turn warms the air above it by conduction and convection. As higher land is further away from the main heat source,
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it is relatively cooler. Also the density of air decreases with height.
• Sunspots- Sunspots are created in the solar outer surface due to periodic disturbances and explosions. The sunspots are cyclic in nature. The increase and decrease of number of sunspots is completed in cycle of 11 years. The amount of insolation received at earth's surface increases and decreases with number of sunspots.
H eat Budget
The Earth's climate is a solar powered system. The earth intercepts only ½ of the billionth fraction of the energy radiated
from the Sun. The Earth in turn radiates back the energy received from the sun in the form of long wave terrestrial radiations. As a result, the earth neither warms up nor does it get cooled over a period of time. It maintains its temperature.
This can happen only because the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation. This is known as heat budget of the earth.
Let the solar energy radiated be taken as 100 units. Out of total incoming radiation entering the earth's atmosphere 35 per cent is sent back to space through scattering by dust particles (6%), reflection from clouds (27%) and from the ground surface (2%), 51 per cent is received by the earth's surface and 14 per cent is absorbed by the atmospheric gases and water vapour.
Incoming shortwave solar radiation: equals to 100 units
a) Amount lost to space through scattering and reflection equals to 35% comprises of
• Clouds = 27%
• Reflected by ground = 2%
• Scattered by dust particles = 6%
b) He at re ce iv ed by ea rt h eq ua ls to 51 % comprises of
• Through direct radiation = 34% • Received as diffuse day light = 17% c) Absorption by the atmospheric gases and
water vapour equals to 14%
After receiving energy from the Sun the Earth also radiates energy out of its surface into the atmosphere through long-wave radiation. As we have seen above 51 per cent of heat is absorbed by the earth. Thus at the ti me of outgoing radiations 23 per cent of the energy is lost through direct long-wave terrestrial radiation (in which 6% absorbed by atmosphere and 17% goes directly to space). About 9 per cent out of 51 per cent is spent in convection and 19 per cent is spent through evaporation. Thus the total energy received by atmosphere from sun and the earth becomes 48 per cent.
The atmosphere receives a total of 14 + 34 = 48 units and this amount is radiated back to space by the atmosphere. The total loss of energy to space thus amounts to 100 units: 35 units reflected by the atmosphere, 17 units lost as terrestrial radiation and 48 units from the atmosphere. In this manner, no net gain or loss of energy occurs in the earth's surface.
Outgoing long-wave terrestrial radiation
a) Reflected by Earth which was equal to 51 per cent as shown above
• 23% from radiation • 9% through convection • 19% through evaporation
b) 48% absorbed in atmosphere moved through radiation back into space.
Although the Earth and its atmosphere as a whole have a radiation balance, there are latitudinal variations. The heat energy is transferred from the lower latitudes to the higher
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latitudes through winds and ocean currents. In the low latitudes (between 400°N and 400°S) heat gained by short wave radiation is far more than the heat loss by long waves through the earth's radiation. In contrast in the higher latitudes more heat is lost by outgoing long wave than it is received in short waves. In view of the imbalances at high and low latitudes, there is large-scale transfer of heat from tropics to high latitudes by atmospheric and oceanic circulation.
Nowadays Green House Gases are disturbing the Earth's heat budget. Carbon dioxide forces the Earth's energy budget out of balance by absorbing thermal infrared energy (heat) radiated by the surface. It absorbs thermal infrared energy with wavelengths in a part of the energy spectrum that other gases, such as, water vapor, do not. Carbon dioxide is a very strong absorber of thermal infrared energy with wavelengths longer than 12-13 micrometers, which means that increasing concentrations of carbon dioxide partially "close" the atmospheric window. In other words, wavelengths of outgoing thermal infrared energy that our atmosphere's most abundant greenhouse gas-water vapor-would have let escape to space are instead absorbed by carbon dioxide.
The absorption of outgoing thermal infrared by carbon dioxide means that Earth still absorbs about 70 percent of the incoming solar energy, but an equivalent amount of heat is no longer leaving. The exact amount of the energy imbalance is very hard to measure, but it appears to be a little over 0.8 watts per square meter.
When increasing greenhouse gas concentrations bumps the energy budget out of balance, it doesn't change the global average
surface temperature instantaneously. It may take years or even decades for the full impact of a forcing to be felt. This lag between when an imbalance occurs and when the impact on surface temperature becomes fully apparent is mostly because of the immense heat capacity of the global ocean. The heat capacity of the oceans gives the climate a thermal inertia that can make surface warming or cooling more gradual, but it can't stop a change from occurring.
However, as long as greenhouse gas concentrations continue to rise, the amount of absorbed solar energy will continue to exceed the amount of thermal infrared energy that can escape to space. The energy imbalance will continue to grow, and surface temperatures will continue to rise. This leads to Global Warming.
Temperature
Temperature is the measurement of available heat energy in a system. It is a measure of hotness and coldness of the body. The atmosphere is not heated directly by insolation or the Sun's rays; rather it is heated from below by the warmed surface of the earth, i.e. from terrestrial radiation. The lower levels of the atmosphere are heated by conduction. The earth's surface is heated
during day time after receiving solar radiation. The air coming in contact with the warmer ground surface is also heated because of transfer of heat from the ground surface through the molecules to the air.
The upper levels of the earth get heated by convection. The air coming in contact with the warmer surface of the earth gets heated and expands in volume. The warmer air rose up and forms vertical circulation of air. This mechanism transports heat from the ground surface to the atmosphere, thus helps in heating up of the atmosphere.
Factors controlling the distribution of temperature:
Latitude: In general, average temperature decreases form the equator towards the poles because the sun rays become more and more
oblique poleward.
• Altitude: the temperature decreases with increasing height from the earth's surface at an average rate of 6.5 °C per 1000 m. The lower layer of air contains more vapour hence it absorbs more heat radiated from the earth's surface than the upper air layers.
• Mountain Ranges: In certain areas of the world the existence of high ranges of mountains acts as formidable barrier to the free circulation of air in the lower reaches of the atmosphere. The Himalayas, for example, prevents the monsoon conditions extending further north into the interior of Asia and prevents the extremely cold anticyclonic winter conditions in Central Asia from penetrating Indian subcontinent. Similarly the Western Cordillera (including the Rocky Mountains) of North America prevents the intrusion of the westerly maritime influence of the Pacific into the heartland of the Prairies and Great Plains.
• Distance from the Sea: The Sea heats up and cools down much more slowly than the
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land. The effects of this phenomenon are more noticeable in the temperate latitudes where the warming effect of the sea particularly affects coastal regions in winter. In general, the sea has a moderating effect on the temperatures of the coastal areas throughout the year. On the other hand, regions deep within the interior of land masses experience extreme temperatures. This phenomenon is known as continentality.
• Ocean Currents: Ocean currents influence the temperature of the coastal regions particularly where onshore winds carry the influence of warm currents towards coastal regions in winter. Cold currents have a cooling effect on the nearby coasts but hav e a lesser effec t than warm currents due to the fact that they often flow below offshore winds.
• Clouds: The presence or absence of clouds in the atmosphere over different regions of the earth's surface has a significant bearing on the temperature of the regions. Clouds have the effect of reducing the amount both of insolation, which reaches the surface of the earth and of outgoing radiation from the earth's surface. As a result, tropical rain forests with dense cloud cover have very little range of temperature, where as, the hot deserts, which have comparatively less cloud cover, have both high diurnal and annual temperature ranges.
• Prevailing Winds: Prevailing winds also affect the temperature conditions of the areas. The moderating effects of oceans are brought to the adjacent lands through winds. On the contrary, off shore winds take the effects of warm or cold currents away from land.
• Local Weather: Local weather comprises different types of storms, cloudiness, precipitation and other weather conditions. In the equatorial regions, despite the vertical rays of the Sun, large amount of cloudiness obstructs the solar radiation from reaching the earth surface. It is due to the clear sky that near the Tropic of Cancer and the Tropic of Capricorn the amount of solar radiation incident on the earth exceeds that reaching the equatorial regions. Thus, in the subtropical high pressure belt the surface water temperature in the oceans is a little higher. Besides, the incidence of daily
afternoon rains in the equatorial regions does not allow the temperatures to rise further, whereas the extremely dry weather and cloudless skies prove helpful in raising the temperatures in the subtropical regions. In the same way in regions of stormy weather the ocean water temperatures are relatively lower.
Concept of I nversi on of Temperat ure
The temperature decreases with the altitudes in the troposphere at an average rate of 6.5 °C per 1000m, this is known as normal lapse rate. But sometimes the temperature increases upward upto a few kilometers from the earth's surface. This is known as inversion of temperature i.e. presence of warm layer of air above the cold layer of air.
Types of inversion of temperature:
1. Ground surface inversion: The most common condition for inversion of temperature is through the cooling of the air near the ground at night. Once the sun goes down, the ground loses heat very quickly, and this cools the air that is in contact with the ground. However, since air is a very poor conductor of heat, the air just above the surface remains warm. Conditions that favour the development of a strong surface inversion are calm winds, clear skies, and long nights. Calm winds prevent warmer air above the surface from mixing down to the ground, and clear skies increase the rate of cooling at the Earth's surface. Long nights allow for the cooling of the ground to continue over a longer period of time, resulting in a greater temperature decrease at the surface. Since the nights in the winter time are much longer than nights during the summer time, surface inversions are stronger and more common during the winter months. During the daylight hours, surface inversions normally weaken and disappear as the sun warms the Earth's surface.
2. Upper air inversion: The thermal upper air inversion is caused by the presence of ozone layer in the stratosphere. The ozone layer absorbs most of the ultraviolet rays radiated from the sun thus the temperature of this layer becomes higher than the other layers.
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Consequences of Temperature Inversions
a) Fog is formed due to the presence of warm air above the cold air. The cold air cools the warm air from below thus forms the tiny droplets around dust particles and smokes during winter season that result in formation of fog. b) Th e ur ba n smog is fo rm ed by th e intensification of fog by pollution. When smog gets mixed with sulphur dioxide it
becomes poisonous and deadly for human beings.
c) Inversion of temperature leads to formation of frost. Frost is economically unfavourable weather phenomenon as it damages fruit orchids and crops.
d) The inversion of temperatures creates anticyclonic conditions thus inhibits rainfall and encourages dry conditions.
small changes in outside air pressure cause the capsule to expand or contract. The size of the aneroid cell is then calibrated and any change in its volume is transmitted by springs and levers to an indicating arm that point to the corresponding atmospheric pressure.
Standard Sea-level Pressure
For climato-logical and meteoro-logical purposes, standard sea-level pressure is said to be 76.0 cm or 29.92 inches or 1013.2 millibars. Scientists often use the kilopascal (kPa) as their
preferred unit for measuring pressure. 1 kilopascal is equal to 10 millibars. Another unit of force some-times used by scien-tists to measure atmo-spheric pressure is the Newton. One millibar equals 100 newtons per square meter (N/m2).
Pressure Bel t s of t he W orl d a) Equatorial Low Pressure Belt:
At the Equator heated air rises leaving a low-pressure area at the surface. This low pressure area is known as equatori al low Ai r Pressure
Air is a mixture of several gases. Gas molecules are in constant state of collision and move freely. Pressure of air at a given place is defined as a force exerted against surface by continuous collision of gas molecules. Air pressure is thus defined as total weight of a mass of column of air above per unit area at sea level. The amount of pressure exerted by air at a particular point is determined by temperature and density which is measured as a force per unit area.
M easurin g At mospheri c Pressure • Torricelli Barometer
The instrument that measures air pressure is called a barometer. The first measurement of atmospheric pressure began with a simple experiment performed by Evangelista Torricelli in 1643. In his experiment, Torricelli immersed a tube, sealed at one end, into a container of mercury. Atmospheric pressure then forced the mercury up into the tube to a level that was considerably higher than the mercury in the container. Torricelli determined from this experiment that the pressure of the atmosphere is approximately 30 inches or 76 centimeters (one centimeter of mercury is equal to 13.3 millibars). He also noticed that height of the mercury varied with changes in outside
weather conditions.
• Aneroid Barometer
It is the most common type barometer used in homes. Inside this instrument is a small, flexible metal capsule called an aneroid cell. In the construction of the device, a vacuum is created inside the capsule so that
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pressure. This area extends between 50°N and
50°S latitudes. The zone shifts along with the northward or southward movement of sun during summer solstice and winter solstice respectively. The pressure belt is thermally induced because the ground surface gets heated during the day. Thus warm air expands, rises up and creates low pressure.
b) Sub-tropical High Pressure Belt:
The warm air risen up at the equator due to heating reaches the troposphere and bend towards the pole. Due to coriolis force the air descends at 30-35º latitude thus creates the belt ofsub-t ropi cal hi gh pressure. The pressure belt is dynamically induced as it owes its origin to the rotation of the earth and sinking and settling of winds. This zone is characterized by anticyclonic conditions which cause atmospheric stability and aridity. Thus the hot deserts of the world are present in this region extending between 25-35 degrees in both the hemisphere.
c) Sub-Polar Low Pressure Belt:
This belt is located between 60-65 degrees latitudes in both the hemisphere. This pressure belt is also dynamically induced. As shown in the figure the surface air spreads outward from this zone due to rotation of the earth thus produces low pressure. The belt is more developed and regular in the southern hemisphere than the northern due to over dominance of water in the former.
d) Polar High Pressure Belt:
High pressure persists at the pole due to low temperature. Thus the Polar High Pressure Belt is thermally induced as well as dynamically induced as the rotation of earth also plays a minor role.
Coriol i s Force
The rot at i on of t he Eart h creat es for ce, termed Cori ol i s force, w hi ch act s upon w i nd. I nst ead o f w i n d b l o w i n g di r ect l y f r o m h i g h t o l o w pressure, the rot at i on of t he Eart h causes w i nd t o be defl ected off course. In t he N ort hern H emi sphere, w i nd i s defl ect ed t o t he ri ght of i t s pat h, whi l e in t he Sout hern H emi sphere it i s defl ected t o t he left . Cori ol i s for ce is absent at t he equat or, and i t s st rengt h i ncreases as one approaches ei t her pol e. Furt herm ore, an i ncrease i n w i nd speed al so result s in a st ronger Cori ol i s for ce, and thus in great er defl ecti on of the w ind.
Shi ft i ng of Pressure Bel t s
By late June, when the sun is overhead at the Tropic of Cancer, the doldrums low pressure belts move significantly northwards from the equator with a resultant shift of other belts in the Northern Hemisphere. Similarly, in late December, when the sun is overhead at the Tropic of Capricorn, the belts move southwards.
W i n d s
When the movement of the air in the atmosphere is in a horizontal direction over the surface of the earth, it is known as the wind. Movement of the wind is directly controlled by pressure.
Horizontally, at the Earth's surface wind always blows from areas of high pressure to areas of low pressure usually at speeds determined by the rate of air pressure change between pressure centres. Wind speed is a function of the steepness or gradient of atmospheric air pressure found between high and low pressure systems. When expressed scientifically, pressure change over a unit distance is called pressure gradient force and the greater this force the faster the winds will blow.
I. Planetary winds:
Planetary winds are major component of the general global circulation of air. These are known as planetary winds because of their prevalence in the global scale throughout the year. Planetary winds occur due to temperature and pressure variance throughout the world.
The pl anet ary w i nds are di scussed bel ow : (a) Trade wind
Winds blowing from the Subtropical High Pressure Belt or horse latitudes towards the Equatorial Low Pressure Belt or the ITCZ are the trade winds. In the Northern Hemisphere, the trade winds blow from the northeast and are known as the Northeast Trade Winds; in the
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Southern Hemisphere, the winds blow from the southeast and are called the Southeast Trade Winds.
The weather conditions throughout the tropical zone remain more or less uniform. This belt is subjected to seasonal variation due to
northward and southward movement of sun. The equatorward part of the trade wind are humid because they are characterized by atmospheric instability thus causes heavy precipitation.
(b) Westerly Wind
The Westerlies are the prevailing winds in the middle latitudes between 35º and 65º latitude, blowing from the high pressure area in the Sub Tropical High Pressure Belt i.e. horse latitudes towards the sub polar low pressure belt. The winds are predominantly from the south-west to north-east in the Northern Hemisphere and from the north-west to south-east in the Southern Hemisphere.
The Westerlies are strongest in the winter season and times when the pressure is lower over the poles, while they are weakest in the summer season and when pressures are higher over the poles. The Westerlies are particularly strong, especially in the Southern Hemisphere, as there is less land in the middle latitudes to obstruct the flow. The Westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the Southern Hemisphere because of its vast oceanic expanse.
(c) Polar Wind
The winds blowing in the Arctic and the Antarctic latitudes are known as the Polar Winds. They have been termed the 'Polar Easterlies', as they blow from the Polar High Pressure belt towards the Sub-Polar Low-Pressure Belts. In the Northern Hemisphere, they blow in general from the north-east, and are called the North-East Polar Winds; and in the Southern Hemisphere, they blow from the south-east and are called the South-East Polar Winds. As these winds blow from the ice-capped landmass, they are extremely cold. They are more regular in the Southern Hemisphere than in the Northern Hemisphere.
II. Periodic Winds:
Land and sea breezes and monsoon winds are winds of a periodic type. Land and sea breezes
occur daily, whereas the occurrence of monsoon winds is seasonal. Following are periodic winds:
(a) Monsoon winds (b) Land and Sea Breeze
(c) Mountain and Valley Breeze
(a) Monsoon Winds
Monsoons are regional scale wind systems that predictably change direction with the passing of the seasons. Like land and sea br eezes, these wind systems are created by the temperature contrasts that exist between the surfaces of land and ocean. However, monsoons are different from land and sea breezes both spatially and temporally. Monsoons occur over distances of thousands of kilometers, and their two dominant patterns of wind flow act over an annual time scale.
Summ er M onsoon
During the summer, monsoon winds blow from the cooler ocean surfaces onto the warmer continents. In the summer, the continents become much warmer than the oceans because of a number of factors. These factors include:
(i) Specific heat differences between land and water.
(ii) Greater evaporation over water surfaces. (iii) Subsurface mixing in ocean basins, which
redistributes heat energy through a deeper layer.
Precipitation is normally associated with the summer monsoons. Onshore winds blowing inland from the warm ocean are very high in humidity, and slight cooling of these air masses causes condensation and rain. In some cases, this precipitation can be greatly intensified by orographic uplift. Some highland areas in Asia receive more than 10 meters of rain during the summer months.
W int er M onsoon
In the winter, the wind patterns reverses, as the ocean surfaces are now warmer. With little solar energy available, the continents begin cooling rapidly as longwave radiation is emitted to space. The ocean surface retains its heat energy longer because of water's high specific heat and subsurface mixing. The winter monsoons bring clear dry weather and winds that flow from land to sea.
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World M onsoonal Cli mate
The Asiatic monsoon is the result of a complex climatic interaction between the distribution of land and water, topography, and tropical and mid-latitudinal circulation. In the summer, a low-pressure centre forms over Northern India and Northern Southeast Asia because of higher levels of received solar insolation. Warm moist air is drawn into the thermal lows from air masses over the Indian Ocean. Summer heating also causes the development of a strong latitudinal pressure gradient and the development of an easterly jet stream at an altitude of about 15 kilometers at the latitude of 25° north. The jet stream enhances rainfall in Southeast Asia, in the Arabian Sea, and in South Africa. When autumn returns to Asia the thermal extremes between land and ocean decrease and the westerlies of the mid-latitudes move in. The easterly jet stream is replaced with strong westerly winds in the upper atmosphere. Subsidence from an upper atmosphere cold low above the Himalayas produces outflow that creates
a surface high-pressure system that dominates the weather in India and Southeast Asia.
Besides, Asian continent, monsoon wind systems also exist in Australia, Africa, South America, and North America.
(b) Land and Sea Breezes:
A land breeze is created when the land is cooler than the water such as at night and the surface winds have to be very light. When this happens the air over the water slowly begins to rise, as the air begins to rise, the air over the surface of the ocean has to be replaced, this is done by drawing the air from the land over the water, thus creating a sea breeze.
A sea breeze is created when the surface of the land is heated sufficiently to start rising of the
air. As air rises, it is replaced by air from the sea; you have now created a sea breeze. Sea breezes tend to be much stronger and can produce gusty winds as the sun can heat the land to very warm temperatures, thereby creating a significant temperature contrast to the water.
(c) Mountain and Valley winds:
Mountain-valley breezes are formed by the daily difference of the thermo effects between peaks and valleys. In daytime, the mountainside is directly heated by the sun, the temperature is higher, air expands, air pressure reduces, and therefore air will rise up the mountainside from the valley and generate a valley breeze. The valley breeze reaches its maximum force at around 2 p.m. After this time, the breeze decreases in
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power and come to a complete stop by sunset. By nightfall, the mountainside region is able to dissipate heat more quickly, due to its higher altitude and therefore temperature drops rapidly. Cold air will then travel down the mountainside from the top and flow into the valley, forming a mountain breeze.
In daytime, valley breezes carry water vapour to the peak, which will often condense into clouds; these are the commonly seen peak and flag clouds. When the mountain breeze travels down and gathers in the valleys, water vapour will condense. Therefore in valley or basin regions, there are usually clouds and fog before sunrise. During late spring or early autumn, the cold air trapped in the valley and basin will often generate
frost. That's why farmers usually plant cold-sensitive plants such as tangerines and coffee on mountainsides.
III. Local Winds
These local winds blow in the various region of the world.
Hot Winds
Sirocco - Sahara Desert Leveche - Spain
Khamsin - Egypt
Harmattan - Sahara Desert Santa Ana - USA
Zonda - Argentina Brick fielder - Australia
Cold Winds
Mistral - Spain and France Bora - Adriatic coast Pampero - Argentina Buran - Siberia Descending Winds Chinook - USA Fohn - Switzerland Berg - Germany Norwester - New Zealand Samun - Persia (Iran) Nevados - Ecuador
JET-STEAMS
The JET STREAMS located in the upper troposphere (9 - 14 km) are bands of high speed winds (95-190 km/hr). The term was introduced in 1947 by Carl Gustaf Rossby. Average speed is very high with a lower limit of about 120 Kms in winter and 50 km per hours in summer. The two most important types of jet streams are thePolar Jet St reams and theSubt ropi cal Jet St reams. They are found in both the Northern and Southern Hemispheres.
Polar Jet Stream: These jet streams are created when cold air from the Polar Regions meets
warmer air from the equator. This temperature gradient as a result forms a pressure gradient that increases wind speed. During the winter, these jet streams bring winter storms and blizzards to the United States and in summer, they become weaker and move towards high latitudes. They are found in both the Northern and Southern Hemispheres.
Sub-Tropical Jet Stream: These jet streams are formed as warm air from the equator moves towards the poles that form a steep temperature incline along a subtropical front that like the polar
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jet streams produce strong winds. In Southeast Asia, India, and Africa, these jet streams help bring about the region's monsoon or rainy climate. As these jet streams warm the air above the Tibetan highlands, a temperature and pressure gradient is formed as the air from the ocean is cooler than that above the continental high lands. This as a result forms on-shore winds that produces the monsoon.
Format i on of Jet St ream
Warm air masses in the south meet cool air masses from the north and create temperature and air pressure gradients. In wind speed, the pressure difference between a high and low pressure zone can be very large, thereby creating high winds. Pressure and temperature differences in the jet stream can be large as a global warm front from the south and a cold front from the north meet.
EL NIÑO AND LA NINA SITUATION
than in the east. Thus forms warmer, deeper waters in the western Pacific and cooler, shallower waters in the east near the coast of South America. The different water temperature of these areas affects the types of weather these two regions experience.
In the east the water cools the air above it, and the air becomes too dense to rise to produce clouds and rain. However; in the western Pacific the air is heated by the water below it, increasing the buoyancy of the lower atmosphere thus increasing the likelihood of rain. This is why heavy rain storms are typical near Indonesia while Peru is relatively dry.
El Niño condition
El Nino happens when weakening trade winds (which sometimes even reverse direction) allow the warmer water from the western Pacific to flow toward the east. This flattens out the sea level, builds up warm surface water off the coast of South America, and increases the temperature of the water in the eastern Pacific.
Normal condition
In general, the water on the surface of the ocean is warmer than at the bottom because it is heated by the sun. In the tropical Pacific, winds generally blow in easterly direction. These winds
tend to push the surface water toward the west also. As the water moves west it heats up even more because it's exposed longer to the sun.
Meanwhile in the eastern Pacific along the coast of South America an upwelling occurs. Upwelling is the term used to describe when deeper colder water from the bottom of the ocean moves up toward the surface away from the shore. This nutrient-rich water is responsible for supporting the large fish population commonly found in this area. Indeed, the Peruvian fishing grounds are one of the five richest in the world. Because the trade winds push surface water westward toward Indonesia, the sea level is roughly half a metre higher in the western Pacific
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An El Nino condition results from weakened trade winds in the western Pacific Ocean near Indonesia, allowing piled-up warm water to flow toward South America.
Since fish can no longer access this rich food source, many of them die off. That is why, these conditions are called "El Nino", or "the Christ Child", which is what Peruvian fisherman call the particularly bad fishing period around December. More importantly, the different water temperatures tend to change the weather of the region.
What happens to the ocean also affects the atmosphere. Tropical thunderstorms are fueled by hot, humid air over the oceans. Hotter the air, the stronger and bigger the thunderstorms. As the Pacific's warmest water spreads eastward, the biggest thunderstorms move with it.
The clouds and rainstorms associated with warm ocean waters also shift toward the east. Thus, rains which normally would fall over the
tropical rain forests of Indonesia start falling over the deserts of Peru, causing forest fires and drought in the western Pacific and flooding in South America. Moreover the Earth's atmosphere reponds to the heating of El-Nino by producing patterns of high and low pressure which can have a profound impact on weather far away from the equatorial Pacific. For instance, higher temperatures in western Canada and the upper plains of the United States whereas colder temperatures in the southern United States. The east coast of southern Africa often experiences drought during El Nino.
La Nina
If, on the other hand, the surface trade winds strengthen and with it the east-west slopes and along with it the east-west oceanic temperature gradients the resulting weather pattern leads to an anti-El Niño, that is often referred to as La Niña. Such events are characterized by a high positive Southern Oscillation Index (i.e. an increased westward pressure gradient over the equatorial Pacific), stronger surface trade winds over the central Pacific, and cooler SSTs in the eastern equatorial Pacific. Such a weather pattern, on the other hand, is associated with increased cyclone activity in the western Pacific, off shore of eastern Australia, the Phillipines, and the western Atlantic region.
CLOUDS AND PRECIPITATION
Moisture, or water vapour, is an extremely important constituent of the atmosphere. Water vapour present in the air is known as Humidity. The nature and amount of precipitation, the amount of loss of heat through radiation from the earth's surface, latent heat of atmosphere etc depend on the amount of water vapour present in the atmosphere.
Water vapour is derived through the process of evaporation from oceans, seas, rivers, lakes etc. Humidity capacity that is content of water vapour in the air is directly positively related with temperature i.e. higher the temperature higher the humidity capacity. Oceanic and coastal areas record higher humidity capacity. It decreases from equator to pole.
Humidity of a place can be expressed in three ways
Absolute Humidity: The measure of water vapour content of the atmosphere which may be
expressed as the actual quantity of water vapour present in a given volume of air is called Absolute Humidity. The absolute humidity is measured in terms of grain per cubic metre air. If the absolute humidity of air at a given place is 10 gm/cu m. it means that 10 grams of water vapour are present in a cubic metre of air. Absolute humidity of the air changes from place to place and from time to time. The ability of air to hold water vapour depends entirely on its temperature. Warm air can hold more moisture than the cold air.
Specific Humidity: Another way to express humidity as the weight of water vapour per unit weight of air or the proportion of the mass of water vapour to the total mass of air is called the specific humidity. Specific humidity is not affected by changes in pressure of temperature.
Relative Humidity: A more useful measure of humidity of the atmosphere is called the relative humidity. This is a ratio expressed as a percentage between the actual quantity of water vapour
